1. Field of Invention
This invention relates generally to video telephone systems employing modems and also applies to modems for digital data transmission.
2. Prior Art
Most modems for transmitting digital data usually transmit one bit per baud. For example, a 2400 baud modem transmits data at 2400 bits per second with each baud or bit being a binary "0" or "1". In this invention several bits of digital data are sent with each baud and the number of bits sent with each baud is multiplied by the baud rate to obtain the bits per second data rate of transmission. The state of the bits for each baud is determined in this modem by the pulse width of each baud, where the pulse width time magnitude is proportional to the binary value of the data. For example, the most significant bit of the data has the maximum impact on the pulse width as determined by its magnitude of 1 or 0. The next most significant bit has one quarter the influence of the most significant bit on the pulse width depending on its magnitude of 1 or 0 and so on for the lesser significant bits.
Use of this modem with a video telephone can significantly increase the data transmission rates across the telephone line. Presently, the most dominant modems used for videophone data transmissions use Amplitude Modulation (AM), Phase Modulation (PM), or vestigial sideband modulation (VSB). These types of modulation are essentially analog in form and require digitization by means of an analog to digital (A/D) converter after the magnitude is recovered, as opposed to digital transmission and recovery. Further, for AM the magnitude varies with each telephone line, so in order to set the amplitude, an introductory signal must be sent to test the line and calibrate the modem. All that is avoided with the pulse width modulation apparatus and method of this invention.
Most present videophones extract the picture information from the synchronization signals and send the picture via a modem device to the far end where the synchronization signals are replaced. In contrast, this invention captures the entire video signal, including the vertical and horizontal synchronization signals and stores them in memory for direct display via a video monitor and transmits all of this data to the receiving video telephone for direct display as well.
The following is a review of some of the key patents in the field of video telephone. U.S. Pat. No. 4,715,059 to COOPER-HART claims a function of resorting the pixels into a smaller pattern of 2000 to 5000 pixels and transmitting them over the line at a rate of 7000 bits per second. The present invention does not perform resorting and transmits the entire frame. U.S. Pat. No. 4,485,400 to Lemelson sends video information in between speech messages, and does not designate a particular format or modem design. U.S. Pat. No. 4,689,661 to Barbieri is concerned with sending multiplexed video signals for more than one camera to the receiving end via radio rather than by telephone lines. The data is transmitted in sinusoidal packets as a subcarrier and employs a specific style of multiplexing and demultiplexing, which is not pulse width modulation. U.S. Pat. No. 4,650,929 to Boerger is for distributing video for a multiplicity of subscribers for a round table discussion by video. Many terminals are involved and the invention consists of a multiplexer, central station control and multiple stations. U.S. Pat. No. 3,950,607 to Southworth deals with bandwidth compression which is a major element and aspect of many video phone systems for conventional telephone lines. The Southworth patent also includes field identification and sends separate signals to identify the field. The present invention does not involve bandwidth compression. U.S. Pat. No. 4,750,039 to Willis is concerned with a freeze frame image to display a "Picture within a Picture". The data taken from the video is compressed but not for the purpose of transmitting the data over the telephone lines and no mention of a modem or means of transmission over telephone lines is mentioned in this patent. U.S. Pat. No. 4,109,276 to Hopkins Jr. concerns itself with claiming means of storing the correct field in memory to retrieve data for a TV station with the correct field synchronization. No mention of use for transmission over narrowband telephone lines of conventional form is mentioned in this patent. U.S. Pat. No. 3,845,250 to Mamil K. Alaily concerns itself with testing for proper video synchronization signals before permitting a phone operator to route a video signal through other media for a video conference. No mention of slowing down or compressing the video data is mentioned in this patent. What is mentioned is pass or fail of a proper NTSC video signal to be passed on over another medium of conveyance. Japanese patent 56-86592 to Shiyouichirou Takayama for a Still Picture Transmitter of Transceiver Type is concerned with the use of a modem with AM, PM, and VSB; however, not with the use of pulse width modulation. U.S. Pat. No. 3,920,901 to Michael A. Boehly concerns itself with dividing frequencies down for particular handling of pixel and sync signals for transmission over some media not carefully described. Even though it is called a video phone, it is not clear that the objective is to send the signal over a narrow band telephone line. The main concern is the method of maintaining synchronization by dividing down the frequencies from a crystal source oscillator to specific frequencies that enhance maintaining synchronization. U.S. Pat. No. 4,654,866 to Deitrich Bottle concerns itself with selecting both a narrow band telephone line for audio communication and another wide bandwidth telephone line for video communication with only one phone number. The video data is not communicated over a narrow band telephone line. Instead a wideband line is selected.
From the above review of prior art there is no use in the prior art of pulse width modulation to transmit digitized or analog picture information on narrowband telephone lines. Nor does the prior art include sending the picture synchronization signals for display along with the picture data.